since the server is not pure anyway, you can do pretty much anything you want. the clients will be very jumpy in area's you change, and of course, they wont see any brushwork you do, which means it will be hard to 'know' where the teleporter is at.

thats been my experiance, i havent done any tests specifically, but i have delt with some maps that were upgraded for better play and they changed brushes and entities, and the client gets very choppy in areas where brushes are added or lost, and entity info is gathered from the server so they are practically the same tho i suppose you lose prediction.

Well, I can certainly confirm that entity changes work.. I've had no trouble adding new spawn points and such, and that causes no noticeable lag. In fact, I believe that cleaning out the unused/nonfunctional SP entities actually improves load times. I also don't believe prediction comes into play for entities that are not tied to brushes, since those are always taken from the server, as far as I know.

I'd never heard of a game allowing these kinds of geometry changes though. I will give it a shot and let you know how things work out.

Okay, so I figured I'd first just try to recompile one of the standard maps with no changes just to make sure everything worked. I converted t1_sour.bsp to a map using q3map2, then tried to recompile I'm getting leaks and such. Is there something special I need to do to recompile the standard maps? I can compile my own maps with no trouble, by the way.

Decompiling (coverting .bsp files to .map files) is a sort of back-door process that's prone to losing map information and screwing things up in general.

You can recompile using the -onlyents flag to change the positions of entities within a map (spawn points, pickups, etc), but I don't think that'll work with teleports since they require brushwork (a trigger_teleport).